The effects of exhaust load and injection timing on particle emission and combustion characteristics were investigated using a single-cylinder gasoline engine equipped with a direct injection system. Particulate matter emission was counted by PPS-M, and the cylinder pressure was measured to analyze the combustion phasing. An exhaust valve was installed to simulate an exhaust load caused by a gasoline particulate filter, and the exhaust load between the exhaust port and after-treatment system was varied with increasing exhaust pressure from 0.1 bar to 0.4 bar as a gauge pressure. Injection timing was swept from BTDC 330 deg to BTDC 210 deg during the intake process to enhance the homogeneity of the mixture.

 The main source of particulate matter at an early injection timing (i.e., BTDC 330 deg) is the fuel film on top of the piston. In addition, the homogeneity of the fuel/air mixture is the most important factor for particle number emission for the late injection case. In-cylinder temperature increased during fuel injection, and the fuel/air mixture became more homogeneous due to the increased internal EGR caused by the exhaust load. As a result, the experimental result showed that a maximum 75% reduction of particle number emission in early injection was achieved by increasing exhaust load from 0.1 bar to 0.4 bar in gauge pressure.